Tantalum capacitors, made from tantalum powder, have been a major contributor to the miniaturization of electronic circuits and have made possible the application of such circuits in extreme environments. Tantalum capacitors typically are manufactured by compressing agglomerated tantalum powder to form a pellet, sintering the pellet in a furnace to form a porous tantalum body (electrode), and then subjecting the porous body to anodization in a suitable electrolyte to form a continuous dielectric oxide film on the sintered body.
Development of powders suitable for making tantalum capacitors has resulted from efforts by both capacitor producers and tantalum processors to delineate the characteristics required for tantalum powder in order for it to best serve in the production of quality capacitors. Such characteristics include specific surface area, purity, shrinkage, pressability, green strength, and flowability.
First of all, the powder should provide an adequate electrode surface area when formed into a porous body and sintered. The ufV/g of tantalum capacitors is proportional to the specific surface area of the sintered porous body produced by sintering a tantalum powder pellet; the greater the specific surface area after sintering, the greater the ufV/g. The specific surface area of tantalum powder is related to the maximum ufV/g attainable in the sintered porous body.
Purity of the powder is an important consideration. Metallic and non-metallic contamination tends to degrade the dielectric oxide film in tantalum capacitors. While high sintering temperatures serve to remove some volatile contaminants high temperatures tend to shrink the porous body reducing its net specific surface area and thus the capacitance of the resulting capacitor. Minimizing the loss of specific surface area under sintering conditions, i.e., shrinkage, is necessary in order to produce high ufV/g tantalum capacitors.
Flowability of the tantalum powder and green strength (mechanical strength of pressed unsintered powder pellets) are also important characteristics for the capacitor producer in order to provide efficient production. The flowability of the agglomerated tantalum powder is essential to proper operation of automatic pellet presses. Sufficient green strength permits handling and transport of a pressed product, e.g., pellet, without excessive breakage.
A `pellet`, as the term is used herein, is a porous mass or body comprised of tantalum particles. Green strength is a measure of a pellet's mechanical strength. The term `pressability` describes the ability of a tantalum powder to be pressed into a pellet. Tantalum powder that forms pellets that retain their shape and have sufficient green strength to withstand ordinary processing/manufacturing conditions without significant breakage have good pressability.
Currently, tantalum powders suitable for use in high performance capacitors are produced by several methods. One powder production method involves chemical reduction, e.g., sodium reduction of potassium fluorotantalate, K.sub.2 TaF.sub.7. In another method, powder is produced by hydriding a melted (typically arc melted or electron beam melted) tantalum ingot, milling the hydrided chips, and dehydriding.
As discussed above, the ufV/g of a tantalum pellet is a function of the specific surface area of the sintered powder. Greater net surface area can be achieved, of course, by increasing the quantity (grams) of powder per pellet; but, cost and size considerations have dictated that development be focused on means to increase the specific surface area of tantalum powder.
One of the methods proposed for increasing the specific surface area of tantalum powder is flattening the powder particles into a flake shape.
Efforts to increase specific surface area by making thinner tantalum flakes have been hindered by concomitant loss of processing characteristics, for example, very thin tantalum flake would be expected to have poor flow characteristics, poor pressability, low green strength and low forming voltages. Thinner flakes are desirable however, because they could make less expensive low density pellets which could be used to make capacitors having high breakdown voltages and low leakage losses.